1. Field of the Invention:
The invention relates to a furnace for heating articles in which the exhaust gases of the combustion chamber are recirculated for preheating of the articles, the furnace having a partially sealed pressure chamber disposed at the front end of the preheating section to prevent the introduction of ambient air into the furnace.
2. State of the Prior Art:
It is common to use furnaces having an elongated heating chamber for heating billets for logs used in extrusion or similar metal working operations. A typical furnace has an elongated heating chamber in which burners are disposed along the length of the furnace to direct burning gases against the billets as they move through the heating chamber. Such furnaces are often of the flame impingement type in which the burner flames envelope the billets within the furnace.
It is known to use the exhaust gases of the combustion chamber to preheat the billets within the furnace. In U.S. Pat. No. 3,837,794, issued Sept. 24, 1974, a billet heating furnace similar to that described above is disclosed in which hot combustion gases are drawn through a preheating zone for heat exchange with the articles passing therethrough. It is also known to exchange the heat of the combustion gases with combustion air and supply this air to a preheating zone as well as to burners in a heating chamber. Such a furnace is disclosed in U.S. Pat. No. 4,022,571, issued May 10, 1977. Additionally, U.S. Pat. No. 3,841,614, issued Oct. 15, 1974, discloses an apparatus for preheating articles in which the hot exhaust gases are supplied through jet nozzles in a preheater zone in order to provide a turbulent flow about the articles being preheated.
In the above-described furnaces, it is important to maintain a pressure balance throughout the length of the furnace. In this way, the introduction of ambient air into the furnace, which reduces the efficiency of operation of the furnace, is minimized. The need for maintaining such a pressure balance is especially important when the furnace includes an exhaust fan near the inlet section. A fan located near the inlet creates a low-pressure zone in the preheater section through which ambient air may be drawn.